1. Field of the Invention
The present invention relates to implants, and, more particularly, to orthopaedic implants.
2. Description of the Related Art
Orthopaedic implants include short-term implants, long-term implants, and non-permanent implants. Short-term implants include implants for the treatment of infection. Long-term implants include total implants for total hip, knee, shoulder, and elbow joints. Non-permanent implants include trauma products such as nails, plates, and external fixation devices.
Regarding short-term implants, when tissue, especially bone, surrounding an orthopaedic implant becomes infected, that implant must typically be removed, the infection must be eliminated, and a new implant (revision implant) is then implanted. The span of time between implant removal and revision implantation can be from several weeks (about 4 weeks) to a few months (approximately 3 months). During this time surgeons currently have two basic options: create temporary implants during surgery with antibiotic bone cement (created with or without the aid of a mold) or use a preformed antibiotic bone cement temporary implant (e.g. Exactech's InterSpace™ Hip and Knee). In either case, antibiotic bone cement is used to deliver antibiotics directly to the site of the infection in the bone. The patient also typically receives IV antibiotics. The shortcomings of such implants are the limited duration in which they deliver a clinically relevant dose of antibiotics, the lack of ability to change antibiotic type or dose during the 4-12 week treatment time, and the limited patient mobility, range of motion, and weight bearing that they allow.
Further, antibiotic cements typically provide useful local antibiotic levels for a duration of less than one week. The treatment time is frequently 6 to 8 weeks. However, beyond one week, the antibiotic cement implants provide no useful amount of antibiotics.
Further, infections can be caused by a great number of bacteria, viruses, yeast, etc. The effectiveness of various antibiotics depends greatly upon what in particular has caused the infection. Thus, in order to treat an infection most effectively, the cause of that infection must be known. The results of cell cultures give this information and indicate which antibiotic and dose will most effectively treat the infection. The samples for culturing are usually collected during surgery. The results of the culture are not known until several days after the surgery. Since the type of antibiotic cement used in current temporary implants must be chosen at or before the time of surgery, the information gained from the cultures cannot be applied to the antibiotics used at the infection site.
Further, one key to a patient recovering from joint surgery with full range of motion in that joint is to encourage movement of that joint. This helps to prevent the formation of scar tissue and stiffening of tissue around the joint. The current options for temporary implants allow limited range of motion and weight bearing at best.
Regarding long-term implants, with regard to bone ingrowth, bone ingrowth into a porous material is sometimes required to provide stability or fixation of an implant to the bone. Examples of this include porous coatings on total joint components, fusion devices (i.e., spinal fusion devices), and bone augmentation components (i.e., tibial wedges).
With regard to resorbtion, resorbtion can occur in the region surrounding a total joint implant for a number of reasons and can lead to implant loosening and subsequent revision surgery. Some causes of resorbtion include:                Stress shielding—Bone tissue requires loading to remain strong and healthy. If an implant does not properly transfer loads to the surrounding bone, regions of bone can resorb.        Lysis due to wear particles—Osteolysis and resorbtion are frequently caused by the body's reaction to wear particles created by the bearing of one total joint component on another.        Osteoporosis or other bone disorders—bone metabolic disorders can also cause the resorbtion of bone.        
With regard to oncology, localized delivery of oncological drugs in the region of tumors may improve results in slowing/halting tumor growth. The ability for localized delivery may also lessen the need/dose of systemic drugs, resulting in fewer side effects.
Regarding non-permanent implants (i.e., trauma implants), such non-permanent implants include nails, plates, and external fixation devices. Nails are temporary, intramedullary devices. They are typically used to treat traumatic fracture. The risk of infection can be high especially in the case of open fractures. With regard to oncology, nails can be used to treat fractures associated with bone tumors. They can also be used to help prevent a fracture where cancer has weakened bone. Plates treat many of the same indications as nails; however plates are applied to the outside of the bone. External fixation devices are a temporary implant that is used to stabilize a fracture. These can be used for days to months. External fixation devices typically include several pins fixed in the bone and extending through the skin to a rigid plate, ring, rod, or similar stabilizing device. These devices carry the added risk of infection due to their extending through the skin. Bacteria can travel along the pins directly to the soft tissue and bone.
What is needed in the art is an orthopaedic implant which includes a reservoir and a plurality of channels leading from the reservoir to deliver at least one therapeutic agent locally to bone or surrounding soft tissue.